In an image display apparatus such as a liquid crystal projector and a digital light processing (DLP) projector, a lamp system including a high pressure discharge lamp such as a metal halide lamp, a xenon lamp, and a high pressure mercury lamp is used.
A conventional high pressure discharge lamp includes an envelope made of fused quartz. The envelope includes a substantially spherical light-emitting part having a discharge space therein, and a pair of sealing parts that are connected to the light-emitting part. In each of the sealing parts, a discharge electrode and an external lead wire are disposed at either ends of a metal foil in a longitudinal direction of the metal foil. In the discharge space, particular discharge gas and mercury are enclosed and the discharge electrodes are disposed to face each other at a fixed interval. Upon lighting of the lamp, light is radiated by the emission of an arc discharge between the pair of discharge electrodes disposed to face each other. The external lead wires are disposed to be partly exposed outside of the sealing parts, and receive power supplied from an external lighting circuit.
The high pressure discharge lamp with the above structure is integrated with a reflector having a reflective surface so as to efficiently emit the light generated by the light-emitting part toward the front. The high pressure discharge lamp and the reflector are used as a lamp unit.
A high pressure discharge lamp has conventionally been demanded to reduce breakdown voltage thereof. In general, breakdown voltage of a high pressure discharge lamp is a high voltage of kV order, and a lighting circuit needs to generate a voltage larger than the breakdown voltage. Therefore, the circuit that generates the high voltage needs to use a large transformer, a highly voltage-resistant electronic part and the like. Also, it is necessary to give adequate consideration of insulation to components such as cables and connectors to which high voltage is applied and peripheral components thereof. Thus, high breakdown voltage of a high pressure discharge lamp is a great obstacle in reducing the size or cost of a lamp system including a lighting circuit. Therefore, reduction in breakdown voltage is demanded.
There has been a conventional approach to reduce breakdown voltage of a high pressure discharge lamp. According to this approach, as shown in the lamp 10X depicted in FIG. 22, an adjacent conductor 21X as a start-up assist member is provided on the outer surface of the lamp 10X in the vicinity of a light-emitting part 100 (For example, see Patent Literature 1). To be specific, the upstream end of the wire-like adjacent conductor 21X is connected to an exposed portion of the external lead wire 102A corresponding to either a sealing part 101A or a sealing part 101B (here, 101A). The adjacent conductor 21X is extended across the light-emitting part 100 toward the sealing part 101B that is opposite to the sealing part 101A so that the adjacent conductor 21X is close to or in contact with the outer surface of the light-emitting part 100 of the lamp 10X. The adjacent conductor 21X is wound around the outer surface of the lamp 10X in the vicinity of the border of the light-emitting part 100 and the sealing part 101B. Upon lighting of the lamp, by applying a voltage to the external lead wire 21X, an intense electric field occurs between the adjacent conductor 21X and the discharge electrode with the opposite polarity (the discharge electrode 104B in FIG. 22) before discharge occurs between the pair of discharge electrodes 104A and 104B in a discharge space 105. Such an intense electric field encourages electrons to emit from the discharge electrode 104B, which reduces breakdown voltage of the lamp 10X. However, even when the above adjacent conductor 21X is used, it is difficult to say that an effect of reducing breakdown voltage of the lamp 10X is adequately obtained.
Alternatively, another approach is to reduce breakdown voltage by enclosing radioactive gas (for example, Kr85) in a light-emitting part. This approach has an effect of adequately reducing breakdown voltage of a lamp. However, it is preferred that there are other approaches in consideration of environmental awareness that has risen in recent years.
In view of the above, in recent years, there has been proposed an approach to further improve an effect of start-up assistance by using a devised structure of an adjacent conductor wire (for example, see Patent Literature 2). Such an approach is to connect the upstream end of an adjacent conductor wire to one of the external lead wires, and put the downstream end of the adjacent conductor wire in contact with substantially only one point of the outer surface of the sealing part. The sealing part is made of fused quartz and encloses a metal foil with a polarity opposite a polarity of the one external lead wire. Upon lighting of the lamp, by applying a voltage to the external lead wires, a corona discharge occurs at the point where the downstream end of the adjacent conductor wire is in contact with the outer surface of the sealing part made of fused quartz before discharge occurs between the pair of discharge electrodes. In accordance with the corona discharge, ultraviolet (UV) occurs and the lamp is irradiated with the UV. Such UV radiation generates photoelectric effect on the surface of the discharge electrodes, and electrons (photoelectrons) are discharged. This encourages the lamp to start discharge. As a result, breakdown voltage is reduced.